In this investigation it is intended to get molecular level understanding to the basis of ion selectivity in the prokaryotic voltage-gated sodium channel. Voltage-gated sodium channels are integral membrane proteins that preferentially pass Na+ ions across the cell mebrane depending on the membrane potential. They are highly conserved throughout evolution. In humans, they regulate many cellular processes, mainly the excitation of nerve and muscle cells. Several diseases of hyperexcitability have been associated with defects in these channels. The current research focuses on the prokaryotic voltage-gated Na+ channel to get crucial insights into the molecular basis underlying the selectivity mechanism of sodium channels and also will shed light on the functional properties of the voltage-gated calcium channels, whose ion conduction pore are believed to be similar to that of sodium channels. The prokaryotic sodium channel shares high sequence similarity to eukaryotic sodium and calcium channels but has several practical advantages over its eukaryotic counterparts in that it is easier to clone into suitable expression vectors and express in large quantities. A combined structural and functional approach will be employed. X-ray crystallographic structure data can reveal the underlying molecular basis for Na+ selectivity. The first approach is thus to crystallize the pore of the channel and determine its three dimensional structure using X-ray crystallography. The emphasis is on the pore region because the voltage sensor component is believed to be highly flexible, which introduces technical difficulties for crystallization. Concurrently, systematic mtagenesis coupled with electrophysiological methods will be employed to tacke the problem from another direction. To achieve these goals the gene has already been cloned from six different bacterial species. Well-expressing clones have been characterized and conditions optimized. Relevance of the research to public health. Many Americans and many more across the globe surfer from several diseases of the muscle, heart or the nervous system that result from mutations in the sodium channel protein. Some of these diseases are epilepsy, long QT syndrome, periodic paralysis, myotonia, etc. If the goals of the propsed research are achieved it would provide a structural and molecular framework for systematic drug design and treatment strategies.